selftests: netfilter: Run nft_audit.sh in its own netns

Don't mess with the host's firewall ruleset. Since audit logging is not
per-netns, add an initial delay of a second so other selftests' netns
cleanups have a chance to finish.

Fixes: e8dbde59

 ("selftests: netfilter: Test nf_tables audit logging")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>